This invention relates to a machine for the treatment of continuous web in the production of a series of (usually) identical areas of the web which are closely spaced along the length of the web. The invention is especially useful in die-cutting to produce separate blanks which can be stripped from the web, for example in the manufacture of printed cardboard boxes, in which case the die cuts may need to be accurately synchronised with pre-printed areas on the web. (If for example boxes and lids are produced alternately from the same web, this will give rise to the case where consecutive blanks are not identical). The invention may also be used in the printing of such areas, and for convenience in this specification, the word "blanks" will be used for both situations.
In order to achieve high speed production, possibly at a rate of thousands of blanks per hour, the treatment is effected by rotary rolls which carry the die-cutting forms or printing stereos (collectively called "forms" herein) and the surface speed of these is matched to that of the web.
In a simple hypotehtical case the web and roll may run at constant speed, but then the blanks will only be closely spaced along the web if the peripheral dimension of the roll (P) is equal to or slightly more than the length (L) of each blank.
The users of such a machine may need to switch from one blank size to another quite frequently, but the only theoretical possibility with such a simple machine would be to change the roll diameter so that factor P of the new roll continued to be equal to or slightly more than the dimension L of the new blank. This is not a practical possibility.
It has therefore been proposed in British Pat. No. 1093723 to duplicate the roll (and for the purpose of explanation of the theory behind this it will be assumed that the rolls then have a periphery equal to 2 P, although this is not a necessity). The two rolls are then synchronised in phase relative to the web and effectively the leading one of each two adjacent blanks is produced by one roll and the trailing one by the other roll. When new blanks of different lengths are to be produced, the new formes are mounted on the rolls and, according to said patent, the speed of the web or the speed of the rolls is varied, once in each cycle, and this is done during the portion of the cycle when the forms are out of contact with the web. Thus if the new length L is greater than before, the web has to be accelerated and then retarded to the matched speed, or the rolls have to be retarded and then accelerated to the matched speed: if the new length L is less than before, vice versa.
Said patent proposes epicyclic gears for roll speed variation but this is complex and may be subject to difficulties in achieving accuracy because of back-lash or play in the gear system. It is also expensive to produce. The object of the invention is to provide new means for roll speed variation which avoid these problems.